fix: sequel migration 19 for mysql

Closes: pact-foundation#33

Author: bethesque

db/migrations/19_make_pact_version_content_sha_not_nullable.rb

@@ -2,9 +2,17 @@
 require_relative 'migration_helper'
 
 Sequel.migration do
-  change do
+  up do
+    PactBroker::MigrationHelper.with_mysql do
+      run("SET FOREIGN_KEY_CHECKS = 0")
+    end
+
     alter_table(:pacts) do
       set_column_not_null(:pact_version_content_sha)
     end
+
+    PactBroker::MigrationHelper.with_mysql do
+      run("SET FOREIGN_KEY_CHECKS = 1")
+    end
   end
 end